The present invention relates to an oilless screw compressor incorporating a heat-exchange for cooling compressed air.
There has been known an oil-free compressor having a pair of male and female screw rotors which can be rotated by timing gears in a contactless and oilless manner so as to compress air. The oil-free compressor has a compressor body for compressing air, and since the temperature of the compressed air discharged from the compressor body becomes high, the compressor is incorporated with a cooling unit for cooling the compressed air.
JP-A-3-290089 discloses a single stage oil-free compressor having such a configuration that a pre-cooler or an after-cooler is incorporated as the cooling unit for cooling the compressed air. In this example, an external cooling water is fed through the cooling unit in order to aim at cooling the compressed air.
JP-A-2001-153080 discloses a two-stage compressor having two compressor bodies. In this compressor, the compressed air from the first stage compressor body is cooled by an intercooler and the compressed air from the second stage compressor body is cooled by an aftercooler, respectively, as cooling units, which are fed thereinto cooling water. Further, JP-A-2006-249934 discloses a two-stage compressor in which the compressed air is cooled by a plate-type heat-exchanger.
In a screw compressor, a power required for compressing air is converted into heat, and accordingly, the temperature of the compressed air rises. The temperature of the compressed air becomes extremely high. As to the oilless screw compressor (oil-free compressor), the temperature of the compressed air discharged from the compressor body comes up to a temperature in a range from about 300 to 350 deg. C. in the case of the single stage type compressor, and in a range from 160 to 250 deg. C. even in the case of the two-stage type compressor.
There have been frequently used, as the cooling unit for cooling the high temperature compressed air in a water cooled compressor, a shell-and-tube type water cooled heat-exchanger (as, for example, disclosed in JP-A-2001-153080) in both single-stage and two-stage type. In the case of the two-stage type compressor, there are arranged individually a heat-exchanger for cooling a low pressure stage compressed air and a heat-exchanger for cooling a high pressure stage compressed air.
It has been difficult to miniaturize the shell-and-tube type water cooled heat-exchanger in view its structure, that is, it has been difficult to greatly miniaturize not only the cooling unit itself in the oilless screw compressor but also the oilless screw compressor unit. JP-A-3-290089 discloses an example utilizing a tube-type heat-exchanger, which is also difficult to be miniaturized, that is, it has been such a configuration that the miniaturization thereof is difficult.
Thus, it has been proposed to use the plate type heat-exchanger, which has a volumetric ratio of about 1/10 to 1/20 in comparison with the shell-and-tube type heat-exchanger, that is, the miniaturization thereof is extremely simple.
However, in the case of using the plate type heat-exchanger for cooling the high temperature compressed air discharged from the compressor body of the oilless screw type compressor, its fitting ports, channel plates, brazed portions between the channel plates and cover plates would be damaged or broken due to thermal fatigue caused by temperature difference. In particular, in a compressor which can be driven in response to the users' demand, upon automatic stopping of the compressor or unload operation (no-load running) thereof, only a slight quantity of compressed air remains in the heat-exchanger, that is, only the cooling water flows through the plate-type heat-exchanger, resulting in high possibility of occurrence of a temperature difference. At this time, since the cooling unit would be damaged or broken within a short time, there has been caused lowering of the reliability of the compressor itself.